Add Menhir/OCamlyacc syntax highlighting

[mnxn]

Addresses #21

Parts of the regex were taken from the Menhir tmLanguage file in https://github.com/hackwaly/vscode-ocaml

Some of the interactions between this grammar and the existing OCaml grammar are unexpected.
For example, everything after List will not be highlighted, but adding ;; after List will make it work fine.

%{
open List
%}
%start <Ast.program> program
%%
program: module_body EOF { Program $1 }

I've deduced that this is a problem with the existing OCaml syntax and I'm working on a much simpler ocaml grammar which I can confirm would fix this problem and other inconsistencies.

I have not found any highlighting issues with the Menhir/OCamlyacc grammar itself.

[rgrinberg]

Looks good to me. @imbsky feel free to merge once you've tested it.

[smorimoto]

Thanks @mnxn for your contribution!